FIELD OF THE INVENTION
The present invention relates to a transponder system which may be used to identify animate or inanimate objects.
Such systems operate by each animate or inanimate object carrying a transponder which responds to electromagnetic signals transmitted by a central interrogator and the transponder transmitting response signals in response to signals received by the interrogator.
A problem arises that the transponder carried by the animate or inanimate object is usually in the form of a tag and may be at any angle or disposition relative to the interrogator antennae. In certain orientations of the tag very little of the signal transmitted by the antennae will be received by the tag. To overcome this problem a system has been proposed in British Patent Specification No. 1599120 with two antennae disposed at a right angle to each other, RF power being switched to each antenna in turn. However a problem with two antennae disposed at right angles is that of siting the antennae at a convenient point. Frequently it is necessary to site the antennae on a wall by which animals or people pass. In this case it is inconvenient to have one antenna projecting from the wall. As a further example the transponder system may be used to detect vehicles with the antennae disposed beneath a roadway. In this case it is highly inconvenient to have one antenna projecting downwardly into the earth beneath the road.